The present invention relates to improvements in an apparatus wherein charged particle beams, which are respectively formed of electrons and negative ions, are simultaneously derived from an identical source so as to irradiate the surface of a specimen therewith. More particularly, it relates to a lens system in the apparatus of this type.
The simultaneous irradiation of the surface of a specimen with an electron beam and a negative ion beam makes it possible to conduct the dynamic structural observation and elemental analysis of the specimen on the surface and in the depth direction thereof.
As a charged particle beam-irradiation apparatus which is used for such purpose, there is one disclosed in U.S. Pat. No. 3,617,739. This apparatus is so constructed that an ion source and an electron source are individually disposed, and that an ion beam and an electron beam are guided to an identical lens system by magnetic deflection means so as to irradiate the surface of a specimen therewith. The lens system in this apparatus is constructed of a hybrid lens in which a magnetic lens and an electrostatic lens are unitarily formed.
Another charged particle beam-irradiation apparatus for the same purpose is disclosed in European Patent Application Publication No. 0003659. This apparatus is so constructed that an electron beam and a negative ion beam are simultaneously derived from an identical source, and that the surface of a specimen is irradiated with the beams by an electrostatic lens.
The construction in which the ion source and the electron source are individually disposed, needs to guide the ion beam and the electron beam to the identical lens system by the use of the magnetic deflection means. With this construction, therefore, it is difficult to remove the deflection distortions of the respective beams. As a result, it becomes difficult to project the ion beam and the electron beam on an identical place. Moreover, since the hybrid lens uses parts of the electrodes of the magnetic lens and the electrostatic lens in common; the principal planes of both the lenses are substantially in agreement. As a result, in a case where such hybrid lens is used as an objective lens, the working distance between the electron beam and the ion beam cannot be varied largely.
In contrast, the construction in which the electron beam and the ion beam are simultaneously derived from the identical source is easy of projecting the respective beams on an idential place. Moreover, since the electrostatic lens is employed as the lens system of the apparatus, the electron beam and the ion beam having equal levels of energy undergo the same focusing actions. As regards the electrostatic lens, however, it is essentially difficult to fabricate a short focus lens. As a result, it is generally difficult to obtain an electron beam having a minute spot diameter.